U.S. patent application number 12/159520 was filed with the patent office on 2009-12-24 for use of metal complexes as emitter in an organic light-emitting component and such a component.
This patent application is currently assigned to NOVALED AG. Invention is credited to Horst Hartmann, Hartmut Yersin.
Application Number | 20090318698 12/159520 |
Document ID | / |
Family ID | 36095764 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090318698 |
Kind Code |
A1 |
Hartmann; Horst ; et
al. |
December 24, 2009 |
Use of Metal Complexes as Emitter in an Organic Light-Emitting
Component and such a Component
Abstract
A HEMT device including a GaN channel structure including a very
thin (Al, In, Ga)N subchannel layer (14) that is disposed between a
first GaN channel layer (12) and a second GaN channel layer (16),
to effect band bending induced from the piezoelectric and
spontaneous charges associated with the (Al, In, Ga)N subchannel
layer. This GaN channel/(Al, In, Ga)N subchannel arrangement
effectively disperses the 2DEG throughout the channel of the
device, thereby rendering the device more linear in character
(relative to a corresponding device lacking the subchannel
(Al5In3Ga)N sub-layer), without substantial loss of electron
mobility.
Inventors: |
Hartmann; Horst; (Dresden,
DE) ; Yersin; Hartmut; (Sinzing, DE) |
Correspondence
Address: |
SUTHERLAND ASBILL & BRENNAN LLP
999 PEACHTREE STREET, N.E.
ATLANTA
GA
30309
US
|
Assignee: |
NOVALED AG
Dresden
DE
|
Family ID: |
36095764 |
Appl. No.: |
12/159520 |
Filed: |
January 24, 2006 |
PCT Filed: |
January 24, 2006 |
PCT NO: |
PCT/US06/02330 |
371 Date: |
November 10, 2008 |
Current U.S.
Class: |
546/4 |
Current CPC
Class: |
C09K 11/06 20130101;
C09K 2211/1092 20130101; H05B 33/14 20130101; H01L 51/0083
20130101; H01L 51/0087 20130101; C09K 2211/1029 20130101; H01L
51/0084 20130101; C09K 2211/185 20130101; H01L 51/0085 20130101;
H01L 51/0086 20130101; H01L 51/5016 20130101; H01L 51/0088
20130101 |
Class at
Publication: |
546/4 |
International
Class: |
C07F 15/00 20060101
C07F015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2005 |
EP |
05028570.9 |
Dec 22, 2006 |
DE |
PCT/DE2006/002330 |
Claims
1. Electronic component comprising an emitter comprising a metal
complex containing 2-(3-thienyl)-pyridine ligands wherein the metal
complex is of general formula 1 or 2: ##STR00001## wherein M is a
heavy metal, X, Y and LL are ligands, which independently of one
another are charged or uncharged and from which essentially no
emission comes, R.sup.1 and R.sup.2 independently of one another
may be present one or more times on the respective cycle and are
independently selected from F, Cl, Br, I, NO.sub.2, CN, a
straight-chain or branched or cyclic alkyl or alkoxy group with 1
to 20 carbon atoms, wherein one or more non-adjacent CH.sub.2
groups may be replaced by --O--, --SiR.sup.3.sub.2--, --S--,
--NR.sup.3-- or --CONR.sup.3-- and wherein one or more H atoms may
be replaced by F, or an aryl or heteroaryl group with 4 to 14 C
atoms, which may be substituted with one or more non-aromatic
radicals R.sup.1 or R.sup.2; wherein a number of substituents
R.sup.1 and/or R.sup.2, both on the same ring and on the two
different rings, may together form a further monocyclic or
polycyclic ring system; R.sup.3 is identical or different each time
it occurs and is selected from H or an aliphatic or aromatic
hydrocarbon radical with 1 to 20 carbon atoms; n=1-3, m and l
independently of one another are 0-4 where 2n+m+l=4 or 6, or p=0-2
where 2n+2p=4 or 6.
2. Electronic component according to claim 1, characterized in that
the heavy metal is selected from Pt(II), Pt(IV), Re(I), Os(II),
Ru(II), Ir(I), Ir(III), Au(I), Au(III), Hg(I), Hg(II) and
Cu(I).
3. Electronic component according to claim 1, characterized in that
X and Y independently of one another are ligands with a single
negative charge or neutral monodentate ligands.
4. Electronic component according to claim 3, characterized in that
X and Y independently of one another are selected from F.sup.-,
Cl.sup.-, Br.sup.-, I.sup.-, CN.sup.-, NCO.sup.-, SCN.sup.-,
R.sup.4S.sup.-, R.sup.4O.sup.-, R.sup.4C.dbd.C.sup.-,
R.sup.4COO.sup.-, NO.sub.3.sup.-, amine, phosphane, arsane,
nitrile, isonitrile, CO, carbene, ethers and thio-ethers, wherein
R.sup.4 is an organic radical with 1 to 15 carbon atoms, preferably
alkyl.
5. Electronic component according to claim 1, characterized in that
LL, preferably with a single negative charge, is a chelating ligand
and/or a cyclometalating ligand.
6. According to claim 5, characterized in that LL is selected from
.beta.-diketonate, .beta.-diketoiminate,
[(pyrazolyl).sub.2H].sup.-,
[(pyrazolyl).sub.2BR.sup.5.sub.2].sup.-, [pyrazolyl.sub.3BH].sup.-,
[pyrazolyl.sub.4B].sup.-, (triazolyl).sub.2BH.sub.2.sup.-,
(triazolyl).sub.3BH.sup.-, (triazolyl).sub.4B.sup.-,
(Ph.sub.2PCH.sub.2).sub.2BR.sup.5.sub.2.sup.-, R.sup.5COO.sup.-,
NO.sub.3.sup.-, diamine, diphosphane, diarsane, dinitrile,
diisonitrile, dialkyl ethers and dialkyl (thio)ethers, wherein
R.sup.5 is an organic radical with 1 to 15 carbon atoms, preferably
alkyl.
7. Electronic component according to claim 1, characterized in that
the metal complex is used as a triplet emitter.
8. (canceled)
9. Electronic component according to claim 1, in the form of an
organic light-emitting diode (OLED).
10. Electronic component according to claim 9, characterized in
that the organic light-emitting diode contains the complex in a
concentration of 2-20 percent by weight, preferably 5-8 percent by
weight, preferably in an electron transport layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a high electron mobility
transistor (HEMT) device.
DESCRIPTION OF THE RELATED ART
[0002] Gallium nitride (GaN) and GaN-based materials have physical
and electronic properties that make them attractive for high
temperature, high power and high frequency microelectronic devices.
These properties include wide bandgap character, low thermal
carrier generation rates, high breakdown field, high electron
mobility and high electron velocity.
[0003] These properties of GaN and GaN-based materials render them
advantageous for use in high electron mobility transistor devices,
characterized by high electron mobilities, superior charge
confinement and high breakdown voltage. Room temperature radio
frequency (2-10 GHz) output power>2 W/mm is enabled by such GaN
and GaN-based materials.
[0004] Conventional HEMTs have a narrow peak in the distribution of
electrons as a function of depth, which results in a sharp peak in
transconductance and poor linearity characteristics. Attempts to
improve the linearity of the device have included fabrication of
HEMTs with AlGaN channels, to spread the electron distribution.
Unfortunately, such devices suffer from significantly reduced
electron mobility, since the two-dimensional electron gas (2DEG) is
predominately contained within the AlGaN alloy and alloy scattering
of the electrons is a significant effect.
[0005] It would therefore be a significant advance in the art to
provide a HEMT device structure that enables control of electron
distribution to produce improved device linearity, without
substantial adverse affect on electron mobility characteristics of
the device.
SUMMARY OF THE INVENTION
[0006] The invention relates in one aspect to a HEMT device
including a GaN channel structure with a very thin (<75
Angstroms) (Al, In, Ga)N subchannel layer, e.g., an AlN or AlGaN
layer, that is disposed between a main GaN channel and a second GaN
channel, to effect band bending induced from the piezoelectric and
spontaneous charges associated with the (Al, In, Ga)N subchannel
layer, to disperse 2DEG throughout the channel of the device and
achieve superior linearity and electron mobility
characteristics.
[0007] As used herein, "(Al, In, Ga)N subchannel layer" in
reference to the intermediate layer between the main GaN channel
layer and the second GaN layer in the HEMT device of the invention
refers to a layer formed of a nitride composition other than GaN
per se, including one or more of aluminum, indium and gallium,
wherein the metal(s) in such nitride composition are in appropriate
stoichiometric relationship (i.e., Al.sub.xIn.sub.yGa.sub.zN
wherein x+y+z=1, and each of x, y and z may range in value from 0
to 1, with the proviso that z.noteq.1). The expression "(Al, In,
Ga)N" therefore includes AlN, AlInN, AlInGaN, AlGaN, InGaN and InN
as alternative species. AlN and AlGaN are particularly preferred
(Al, In, Ga)N species in the broad practice of the invention. It
will be appreciated that the channel and subchannel layers in the
practice of the invention preferably form one channel, and that the
respective layers may be referred to herein as channel
structures.
[0008] In one aspect, the invention relates to a HEMT device,
comprising:
[0009] a lower GaN channel layer;
[0010] an intermediate (Al, In, Ga)N subchannel layer; and
[0011] an upper GaN channel layer,
wherein the intermediate (Al, In, Ga)N subchannel layer has a
thickness not exceeding 75 Angstroms.
[0012] Another aspect of the invention relates to a HEMT device
with a channel structure including a (Al, In, Ga)N subchannel layer
disposed between a first GaN channel layer and a second GaN channel
layer.
[0013] A further aspect of the invention relates to a DHFET device,
comprising:
[0014] a lower GaN channel layer;
[0015] an intermediate (Al, In, Ga)N subchannel layer; and
[0016] an upper GaN channel layer,
wherein the intermediate (Al, In, Ga)N subchannel layer has a
thickness not exceeding 75 Angstroms.
[0017] Yet another aspect of the invention relates to a method of
improving linearity without substantial loss of electron mobility
in a HEMT device, including fabricating the HEMT device with a
channel structure including first and second GaN channel layers and
a (Al, In, Ga)N subchannel layer therebetween.
[0018] Other aspects, features and embodiments of the invention
will be more fully apparent from the ensuing disclosure and
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic representation of an HEMT device
structure according to one embodiment of the present invention.
[0020] FIG. 2 is a band diagram of the device of FIG. 1.
[0021] FIG. 3 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a structure including a 3 nm
thick GaN channel and a 0.2 nm AlN undoped subchannel.
[0022] FIG. 4 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a structure including a 3 nm
thick GaN channel and a 0.3 nm AlN undoped subchannel.
[0023] FIG. 5 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a structure including a 4 nm
thick GaN channel and a 0.2 nm AlN undoped subchannel.
[0024] FIG. 6 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a structure including a 4 nm
thick GaN channel and a 0.3 nm AlN undoped subchannel.
[0025] FIG. 7 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a structure including a 4 nm
thick GaN channel and a 0.3 nm AlN layer doped with silicon at a
doping density of 10.sup.13 atoms/cm.sup.2.
[0026] FIG. 8 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a device structure including a
5 nm thick AlGaN channel, containing 5% aluminum in the AlGaN
channel material.
[0027] FIG. 9 is a graph of conduction band edge, E.sub.c, in
electron volts (eV) and carrier concentration, in cm.sup.-3, as a
function of depth in Angstroms, for a device structure including a
2 nm GaN channel, a 0.3 nm undoped AlN layer, a 3 nm GaN channel
and a 0.3 nm undoped AlN layer.
[0028] FIG. 10 is a schematic representation of a HEMT device
structure including an AlN barrier layer and characterized by a
sheet mobility of .about.330 .OMEGA./square.
[0029] FIG. 11 is a schematic representation of a HEMT device
structure including an AlGaN channel and an AlN barrier layer and
characterized by a sheet mobility of .about.610 .OMEGA./square.
[0030] FIG. 12 is a schematic representation of a HEMT device
structure including an AlN subchannel and an AlN barrier layer and
characterized by a sheet mobility of .about.460 .OMEGA./square.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
THEREOF
[0031] The present invention relates to a HEMT device including a
GaN channel structure with a very thin (Al, In, Ga)N subchannel
layer that is disposed between a main GaN channel and a second GaN
channel, e.g., below a main GaN channel and above a second GaN
channel, to effect band bending induced from the piezoelectric and
spontaneous charges associated with the AlInGaN subchannel layer.
This GaN channel and (Al, In, Ga)N subchannel arrangement
effectively disperses the 2DEG throughout the channel of the
device, thereby rendering the device more linear in character
(relative to a corresponding device lacking the (Al, In, Ga)N
subchannel layer) without substantial loss of electron mobility
characteristic of the GaN-based device.
[0032] To achieve such band bending, dispersal of 2DEG throughout
the channel, high electron mobility and improved linearity, the
(Al, In, Ga)N subchannel layer is desirably as thin as possible,
preferably being of monolayer or near-monolayer thickness, and in
any event not exceeding 75 Angstroms in thickness, and more
preferably not exceeding 60 Angstroms in thickness.
[0033] It will be appreciated that in specific embodiments of the
invention, the thickness of the (Al, In, Ga)N subchannel layer can
vary depending on the particular (Al, In, Ga)N subchannel material
being employed. In one embodiment, in which the (Al, In, Ga)N
subchannel layer is constituted by AlN, such AlN subchannel layer
preferably does not exceed 20 Angstroms in thickness, and may for
example be 6 Angstroms or less in thickness. In another embodiment
in which the (Al, In, Ga)N subchannel layer is formed of AlGaN or
AlInGaN, the subchannel thickness preferably does not exceed 50
Angstroms. The choice of a specific material and thickness for the
(Al, In, Ga)N subchannel layer in a given device application of the
invention, can readily be made by those of ordinary skill in the
art, based on the disclosure herein, without undue
experimentation.
[0034] By the utilization of the (Al, In, Ga)N subchannel layer
structure as an intermediate layer in a GaN channel structure
formed of overlying and underlying GaN channel layers, the
inventive device achieves the beneficial effects realized by AlGaN
channels in GaN-based devices, but without the detrimental effect
of the high degree of alloy electron scattering that is typical of
such AlGaN channel GaN devices.
[0035] In one specific embodiment, the subchannel layer in the HEMT
device of the invention is formed of AlN or AlGaN, which can be
doped or undoped in character, e.g., doped with indium. In other
embodiments, the subchannel layer is formed of AlInN or AlInGaN,
with the stoichiometric composition of such material preferably
being selected to provide a subchannel layer that is substantially
lattice matched (within 5% of the lattice coefficient value) to the
GaN channel layers adjacent to the respective surfaces of the
subchannel layer.
[0036] In a particularly preferred embodiment, AlN is used as the
material of the subchannel layer. In HEMT structures utilizing AlN
subchannel layers in accordance with the invention, the thickness
of the AlN subchannel layer can be any suitable thickness effecting
the aforementioned band bending, e.g., typically being in a range
of from 1 Angstrom to 30 Angstroms in thickness, more preferably in
a range of from 2 Angstroms to 10 Angstroms, and most preferably in
a range of from 2 Angstroms to 8 Angstroms. As a specific example,
subchannel thicknesses on the order of 4 Angstroms have been
advantageously employed.
[0037] In HEMT device structures utilizing AlGaN subchannel layers
in accordance with the invention, the thickness of the AlGaN
subchannel layer is generally greater than the thickness of a
corresponding subchannel layer formed of AlN, e.g., thicker by the
inverse of the aluminum fraction. Accordingly, in specific
embodiments, AlGaN subchannel layers can be employed at thickness
in a range of from 2 to 60 Angstroms, with a thickness range of
from 4 Angstroms to 20 Angstroms being more preferred, and
thickness in a range of from 4 Angstroms to 16 Angstroms being most
preferred. The stoichiometric composition of the AlGaN subchannel
material can be widely varied within the formula
Al.sub.xGa.sub.1-xN, wherein 0<x<1. In one preferred
embodiment, for example, the AlGaN subchannel layer has the
stoichiometric formula Al.sub.0.5Ga.sub.0.5N.
[0038] Other specific variations of (Al, In, Ga)N subchannel layer
compositions can be employed, at various specific thicknesses,
within the broad practice of the present invention. For example,
the subchannel can be formed of AlInN, with the stoichiometric
composition appropriately selected for lattice matching to GaN,
e.g., a stoichiometric composition of Al.sub.0.83In.sub.0.17N.
[0039] The thickness of the subchannel layer in the GaN channel
HEMT device of the invention is of fundamental importance, in
providing appropriate band bending and linearity and electron
distribution characteristics. If the subchannel layer is
excessively thin, not enough band bending will be achieved to
simultaneously provide superior linearity and superior electron
distribution in the channel. Further, if the subchannel layer is
excessively thick, it becomes disproportionately susceptible to the
presence of morphological artifacts that render it non-continuous
in character. Additionally, all or substantially all of the
electrons in the channel remain in the bottom channel layer.
[0040] The thickness of the subchannel controls the effective band
offset of the respective GaN channel layers, determining the
distribution of electrons between the respective upper and lower
GaN channels.
[0041] Typically, for good linearity and electron mobility
characteristics, it is desirable to have more electrons in the top
(overlying) GaN channel layer, in relation to the lower
(underlying) GaN channel layer. The electron distribution between
the two GaN channel layers can be efficiently controlled by choice
of appropriate thickness of the (Al, In, Ga)N subchannel layer and
thickness of the upper GaN channel. In preferred practice, the top
channel layer should contain at least slightly more electrons than
the lower channel layer, with effective distributions ranging from
such slight excess of electrons in the top channel layer to an
order of magnitude more electrons than in the lower GaN channel
layer.
[0042] As indicated, the (Al, In, Ga)N subchannel layer may be
doped or undoped in character. Suitable dopant species for such
purpose include, without limitation, silicon (Si) and germanium
(Ge). In general, it is desirable to avoid use of oxygen as a
dopant species, since it may form DX centers in the subchannel that
will deleteriously affect the device performance.
[0043] Doping may be selectively applied to modify the electronic
profile of the channel, and to minimize ionized impurity
scattering.
[0044] Doping densities may be of any suitable character, as
readily determinable within the skill of the art without undue
experimentation, based on the present disclosure. Typical doping
densities for Si and Ge can be in a range of from 10.sup.12
atoms/cm.sup.2 to 2.times.10.sup.13 atoms/cm.sup.2.
[0045] The thicknesses of the upper GaN layer and the bottom GaN
layer (above and below the subchannel layer respectively) may be of
any suitable thickness appropriate to the specific HEMT device
structure. For example, when AlN subchannel layers are utilized,
the upper GaN channel layer typically will be greater than 10
Angstroms in thickness, and the bottom GaN layer typically will be
greater than 100 Angstroms in thickness. As another example, when
AlGaN nucleation layers are utilized, the thickness of the lower
GaN layer may be as low as 30 Angstroms in thickness. Thicknesses
of the respective GaN layers of the channel structure in specific
applications of the invention will be readily determinable within
the skill of the art, based on the disclosure herein. By way of
further illustrative example, in one specific embodiment, the lower
GaN channel layer has a thickness of 1-2 .mu.m, the intermediate
AlN subchannel layer has a thickness of 0.2-0.4 nm, and the upper
GaN channel layer has a thickness of 3-5 nm.
[0046] The channel/subchannel structure of the invention provides
significant flexibility and degrees of freedom in relation to
channel structures of the prior art. The thicknesses of the GaN
channel layers in relation to the subchannel layer thickness can be
varied significantly to achieve specific desired physical
conformations and performance characteristics. For example, to
achieve similar charge in the bottom GaN channel, the upper GaN
channel layer may be made thicker and an AlN subchannel layer may
be made thinner, while achieving a same or similar overall
conformation and performance. Alternatively, n-type doping levels
can be reduced in the device structure, to achieve the same
result.
[0047] The channel structure of the present invention may be
deployed in any suitable HEMT device design, e.g., a strain
balanced HEMT providing confinement for the bottom channel layer.
In one embodiment, the HEMT device is an AlN barrier HEMT.
[0048] The gallium nitride channel layers may be formed using any
appropriate process or technique. For example, such layers may be
formed by vapor phase techniques in which reactant gas species
(e.g., ammonia, trimethylgallium) enter a growth reactor in which
the substrate is disposed. The reactant gas species can be passed
over the substrate to deposit an epitaxial film (e.g., of GaN
incorporating nitrogen from ammonia and gallium from
trimethylgallium). The process may be carried out at appropriate
temperature (e.g., a temperature in a range of from 500.degree. C.
to 1200.degree. C., or in a narrower specific temperature range of
from 700.degree. C. to 1100.degree. C., or in an even narrower
range of from 900.degree. C. to 1000.degree. C. The pressure in the
reactor may be maintained at an appropriate level (e.g., in a range
of from 20 to 950 millibar). The (Al, In, Ga)N subchannel layer may
be formed using any suitable technique or a process known in the
art for formation of monolayer or near-monolayer films, such as
MBE, MOCVD, ALE or the like, and appropriate reagents such as those
mentioned above, trimethylaluminum, trimethylindium, etc. The
substrate can be a wafer of gallium nitride, silicon carbide,
aluminum nitride, aluminum gallium nitride, sapphire, diamond,
silicon, etc.
[0049] In a specific embodiment, the channel layer structure of the
invention is employed in a double heterojunction field effect
transistor (DHFET), e.g., a DHFET device in which the subchannel
layer is formed of AlGaN.
[0050] Referring now to the drawings, FIG. 1 is a schematic
representation of a HEMT device structure according to one
embodiment of the present invention.
[0051] The HEMT device structure shown in FIG. 1 includes a
substrate 10, which may be of any suitable type, e.g. a
homoepitaxial GaN substrate, or alternatively a heteroepitaxial
substrate formed of silicon carbide, aluminum nitride, diamond,
sapphire, silicon, or other appropriate material. Although not
shown, the substrate 10 may include a nucleation layer, e.g., of
AlN or other suitable material, at a thickness that can for example
be on the order of 2000 Angstroms. Further, strain compensation
layers can be employed as necessary or desired for heteroepitaxial
substrates. Such nucleation layers and strain compensation layers
are well known in the art and require no detailed description here,
and may advantageously form part of substrate or buffer layers in
device structures of the present invention.
[0052] Overlying the substrate 10 including optional nucleation
and/or strain compensation layer(s), is a GaN buffer layer 12, on
which (Al, In, Ga)N subchannel layer 14 has been formed. By such
arrangement, the GaN buffer layer 12 defines a lower channel region
in the vicinity of (Al, In, Ga)N subchannel layer 14. Overlying the
(Al, In, Ga)N subchannel layer 14 is an upper GaN channel layer 16
that defines an upper channel region in the vicinity of the
subchannel layer 14. The subchannel layer in this illustrative
embodiment can be formed of AlN, for example, or alternatively it
can be formed of AlGaN or other suitable (Al, In, Ga)N subchannel
layer material.
[0053] Overlying the upper GaN channel layer 16 is an optional AlN
barrier 18. Top layer 20, overlying the upper GaN channel layer 16
and optional AlN barrier 18, is an Al.sub.xIn.sub.yGa.sub.zN cap
layer, formed for example of AlGaN or alternatively of GaN, on the
upper surface of which can be disposed conventional source, gate
and drain elements (not shown in FIG. 1). The barrier layer 18 and
cap layer 20 can be widely varied in composition and conformation,
as well as in processing (e.g., doping, recessing, passivation,
etc.). In the Al.sub.xIn.sub.yGa.sub.zN cap layer, each of x, y and
z has a value of from 0 to 1 inclusive, with x+y+z=1, and each of
x, y and z can optionally vary with depth, so that an AlN/AlGaN/GaN
cap or a graded AlInGaN layer may be present, in specific
embodiments.
[0054] The (Al, In, Ga)N subchannel layer 14 in FIG. 1 is desirably
as close to a monomolecular layer as possible, e.g., 1, 2 or 3
molecular layers in thickness, and most preferably is of monolayer
thickness across the full area extent of the top surface of the
lower GaN channel layer 12. The (Al, In, Ga)N subchannel layer may
for example be formed of AlN and have a thickness on the order of
2.5 to 5 Angstroms, and the upper GaN channel layer 16 may have a
channel thickness on the order of 30-50 Angstroms. The (Al, In,
Ga)N subchannel layer can be formed by any suitable growth
methodology.
[0055] As discussed herein above, it is desirable to keep the (Al,
In, Ga)N subchannel layer as thin as possible, in order to maintain
suitable electron density in the lower GaN channel layer and to
minimize the negative spike in electron density in the vicinity of
the AlN subchannel layer. It may also be beneficial to have a very
low, e.g., monolayer, thickness in the subchannel, to achieve
reduced alloy and roughness scattering, as well as reproducibility
of the subchannel dimensions by deposition techniques such as
atomic layer epitaxy (ALE).
[0056] The structure shown in FIG. 1 may be varied in relative
thicknesses of component layers as may be necessary or desired in a
specific application to achieve desired electron density and output
performance. For example, by making the overlying GaN channel layer
16 thinner, the negative spike in electron density (associated with
the subchannel layer) is rendered less severe, and electron density
in the underlying GaN channel layer increases. One of skill in the
art can readily determine the appropriate relative thicknesses of
the component layers of the HEMT device as necessary or desirable
in a specific device application, by empirical structures of the
present invention.
[0057] FIG. 2 is a band diagram of the device of FIG. 1, showing
that the AlN subchannel layer gives a step in the energy band and
some spill-down of charge, with the upper GaN channel layer of
higher electron content in relation to the lower GaN channel layer
(GaN channel #2).
[0058] FIG. 3 is a graph of conduction band edge (curve A),
E.sub.c, in electron volts (eV), and carrier concentration (curve
B), in cm.sup.-3, as a function of depth, in Angstroms, for a
structure including a 3 nm thick GaN channel and a 0.2 nm AlN
undoped subchannel.
[0059] The FIG. 3 profile shows the upper GaN channel layer
electron density having a roughly parabolic shape with a peak of
about 5E19 cm.sup.-3 at a depth of about 270 Angstroms, and the
lower GaN channel layer electron density exhibits a peak of about
2E18 cm.sup.-3 at a depth of about 310 Angstroms, with the AlN
subchannel layer being centered at a depth of about 290 Angstroms
in the structure.
[0060] FIG. 4 is a graph of conduction band edge (curve A),
E.sub.c, in electron volts (eV), and carrier concentration (curve
B), in cm.sup.-3, as a function of depth, in Angstroms, for a
structure including a 3 nm thick GaN channel and a 0.3 nm AlN
undoped subchannel. The profile shapes for the upper and lower GaN
channel layers in FIG. 4 as compared to FIG. 3 shows that an
increase in the subchannel thickness by 50% (0.3 nm vs. 0.2 nm)
effects a more uniform distribution of charge between the
respective channel layers, with the upper GaN layer nonetheless
retaining more charge than the lower GaN channel layer.
[0061] FIG. 5 is a graph of conduction band edge (curve A),
E.sub.c, in electron volts (eV), and carrier concentration (curve
B), in cm.sup.-3, as a function of depth, in Angstroms, for a
structure including a 4 nm thick GaN channel and a 0.2 nm AlN
undoped subchannel.
[0062] In relation to the conduction band graphs of FIGS. 3 and 4,
FIG. 5 shows the effect of increased thickness of the GaN channel
layer. In the FIG. 5 structure, the upper GaN channel layer has
33.3% greater thickness (i.e., a 4 nm thick GaN channel layer) than
the structures of FIGS. 3 and 4. Such increase in upper channel
thickness increases the charge of the upper channel layer in
relation to the charge of the lower channel layer.
including a 4 nm thick GaN channel and a 0.3 nm AlN undoped
subchannel.
[0063] Comparing FIGS. 5 and 6, it is seen that increasing the
subchannel AlN layer thickness increases the amount of charge in
the lower channel GaN layer.
[0064] FIG. 7 is a graph of conduction band edge (curve A),
E.sub.c, in electron volts (eV), and carrier concentration (curve
B), in cm.sup.-3, as a function of depth, in Angstroms, for a
structure including a 4 nm thick GaN channel and a 0.3 nm AlN doped
with silicon at a doping density of 10.sup.13 atoms/cm.sup.2.
[0065] The effect of silicon doping of the subchannel layer is
shown in FIG. 7 as altering the relative charge of the respective
upper and lower GaN channel layers, so that they are more nearly
equal to one another, but with the upper GaN channel layer
containing slightly more charge than the lower GaN channel layer.
Total charge also is increased relative to an undoped
structure.
[0066] FIG. 8 is a graph of conduction band edge (curve A),
E.sub.c, in electron volts (eV), and carrier concentration (curve
B), in cm.sup.-3, as a function of depth, in Angstroms, for a
device structure including a 5 nm thick AlGaN channel, containing
5% aluminum in the AlGaN channel material.
[0067] FIG. 9 is a graph of conduction band edge (curve A),
E.sub.c, in electron volts (eV), and carrier concentration (curve
B), in cm.sup.-3, as a function of depth in Angstroms, for a device
structure including a 2 nm GaN channel, a 0.3 nm undoped AlN layer,
a 3 nm GaN channel and a 0.3 nm undoped AlN layer.
[0068] The features and advantages of the invention are more fully
shown with respect to the following non-limiting examples.
Examples
[0069] Referring now to the further drawings FIGS. 10-12, FIG. 10
is a schematic representation of a HEMT device structure including
an AlN barrier layer and characterized by a sheet mobility of
.about.330 .OMEGA./square, FIG. 11 is a schematic representation of
a HEMT device structure including an AlGaN channel and an AlN
barrier layer and characterized by a sheet mobility of
.about.4601/square.
[0070] In each of FIGS. 10-12, the thicknesses of the respective
layers in the device structure are set out in appropriate
dimensional units of microns (.mu.m) or nanometers (nm).
[0071] Each of the AlN barrier HEMT structures in FIGS. 10-12 was
grown under substantially the same growth conditions as the others.
All epitaxial III-Nitride layers were deposited by low pressure
MOCVD (metalorganic chemical vapor deposition). Precursors used
were trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia
(NH.sub.3). The carrier gas was a mixture of hydrogen and nitrogen.
The growth temperature was approximately 1000.degree. C.
[0072] FIG. 10 schematically depicts an AlN barrier HEMT. This
device exhibited enhanced mobility and reduced sheet resistivity of
about 330 .OMEGA./square attributable to the thin AlN barrier. The
substrate was a 350 .mu.m thick high-purity semi-insulating (HPSI)
4H--SiC substrate, on which was deposited an AlN nucleation layer
at a thickness of 200 nm. Overlying the nucleation layer is a 1
.mu.m thick GaN:Fe film, on which in turn was deposited a 1 .mu.m
thick layer of gallium nitride. On such GaN layer was deposited a
0.4 nm thick AlN barrier layer, which in turn was capped with a 25
nm thick layer of Al.sub.0.26Ga.sub.0.74N. This AlN barrier HEMT
had a sheet resistivity of approximately 330 .OMEGA./square.
[0073] FIG. 11 is a schematic representation of an AlGaN channel
AlN barrier HEMT device, which had a same layer sequence as the
HEMT device structure in FIG. 10, except that an additional layer
was present in the FIG. 11 structure, between the 1 .mu.m thick GaN
layer and the 0.4 nm AlN barrier layer. This additional layer was a
6 nm Al.sub.0.06Ga.sub.0.94N channel layer. This AlGaN channel AlN
barrier HEMT had a sheet resistivity of approximately 610
.OMEGA./square. In relation to the FIG. 10 HEMT, the electron
mobility in the FIG. 11 device was significantly reduced, and the
sheet resistivity was increased by about 85%, an increase
consistent with higher alloy scattering of electrons in the ternary
channel.
[0074] FIG. 12 is a schematic representation of an AlN subchannel
AlN barrier HEMT. This device structure had the same layer sequence
as the HEMT device structure shown in FIG. 10, except that two
additional layers were present in the FIG. 12 structure, between
the 1 .mu.m thick GaN layer and the 0.4 nm AlN barrier layer. These
additional layers were a 0.3 nm thick
[0075] In relation to the HEMT device structure of FIG. 11, the
HEMT device structure of FIG. 12 replaced the AlGaN channel of the
FIG. 11 device with a GaN channel layer and an AlN subchannel
layer. The HEMT device of FIG. 12 exhibited a sheet resistivity of
approximately 460 .OMEGA./square, still greater than the AlN
barrier HEMT of FIG. 10, but significantly better (approximately
25% lower) than the AlGaN channel structure HEMT of FIG. 11.
INDUSTRIAL APPLICABILITY
[0076] The present invention provides a HEMT device structure that
enables control of electron distribution to produce improved device
linearity, without substantial adverse affect on electron mobility
characteristics of the device, with a GaN channel and (Al, In, Ga)N
subchannel arrangement that effectively disperses the 2DEG
throughout the channel of the device, thereby rendering the device
more linear in character (relative to a corresponding device
lacking the (Al, In, Ga)N subchannel layer) without substantial
loss of electron mobility. The device structure of the invention
has application in wireless communications, low noise amplifiers,
V-band power amplifiers, and millimeter-wave signal processing.
* * * * *